ANOMALOUS DIFFUSION OF PROBE MOLECULES IN POLYSTYRENE - EVIDENCE FOR SPATIALLY HETEROGENEOUS SEGMENTAL DYNAMICS

A holographic fluorescence recovery after photobleaching technique has been used to measure the translational motion of tetracene and rubren e (tetraphenyltetracene) in polystyrene (PS) from T-g - 10 K to T-g 100 K (T-g = 373 K). At 363.5 K, the translational motion of tetracene is not diffusive; i.e., the mean-square displacement is not linear in time on the length scale (60 nm) and time scale (10 h) of the experim ent. Above 370 K, diffusive transport is observed for both probes with diffusion coefficients in the range of 10(-6)-10(-15.5) (cm(2)/s. The observed diffusion coefficients have a significantly weaker temperatu re dependence than rotation times for these two probes in PS. At low t emperatures, the root-mean-square displacement for tetracene approache s 60 nm in one rotational correlation time. It is not necessary to con clude, however, that individual probe molecules translate large distan ces without rotating. All these observations can be rationalized witho ut invoking a change in the local relationship between rotation and tr anslation if the local dynamics of PS are assumed to be spatially hete rogeneous.